The present invention relates to a method of controlling a fixing unit which is installed in an image forming apparatus and constituted by a fixing roller and a pressing roller.
An electrophotographic copier, facsimile apparatus or like image forming apparatus usually includes a fixing unit which includes a fixing roller and a pressing roller. It is a common practice to control the fixing temperature of the fixing roller to a secure fixing temperature so that a certain fixing ratio may be guaranteed. The words "secure fixing temperature" refer to a lower limit at temperatures above which a toner image can be satisfactorily fixed. Generally, when a fixing unit is operated in a continuous copy mode, heat is absorbed by papers which are sequentially fed to the unit with the result that the fixing temperature is lowered. In such a condition, power has to be supplied to the fixing roller. A problem which arises here is that when the power which can be supplied is limited as is the case with a power supply for family use, a continuous copy mode operation or the like is apt to reduce the power available to such an extent that the temperature of the fixing roller becomes lower than the secure fixing temperature. This frequently occurs particularly in a high-speed electrophotographic copier. It has been customary to cope with the drop of the fixing roller temperature beyond the secure fixing temperature by interrupting the copying operation or reducing the copying rate, i.e., by reducing the heat which is absorbed by papers to restore the fixing roller temperature to the secure fixing level. This kind of scheme is undesirable because both the interruption of operation and the reduction of copying rate lower the copying efficiency.
In the light of this, Japanese Laid-Open Patent Publication No. 55-74573, for example, discloses a method which detects a difference in fixing ratio between the time at which a warm-up period of the fixing unit expires and the time to follow. This scheme, however, is not fully satisfactory.